# Change variable outside of local scope

**URL:** <https://discourse.julialang.org/t/change-variable-outside-of-local-scope/81645>\
**Category:** General Usage\
**Created:** [May 25, 2022, 12:15pm UTC](https://discourse.julialang.org/t/change-variable-outside-of-local-scope/81645 "2022-05-25T12:15:31Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![IljaK91](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iljak91/32/44301_2.png) [@IljaK91](https://discourse.julialang.org/u/IljaK91)\
**Post date:** [May 25, 2022, 12:15pm UTC](https://discourse.julialang.org/t/change-variable-outside-of-local-scope/81645/1 "2022-05-25T12:15:31Z")

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Hi everyone,

I try to solve a model and would like to update the parameters of the model with each iteration.

For instance, I have a function like this:

```julia
function solve_GE(par::Pars)
    res = zeros(2)
    f!(x) = residuals_GE(x, res; par)
    initial_x = [1., 1.]
    sol = nlsolve(f!, initial_x)
    return sol
end

```

The function `residuals_GE` takes two parameters, updates the struct `par`, and solves the model given the parameters. Afterward, two residuals are computed that follow from the model’s solution.

My question is whether I can keep the struct par that was computed inside `residuals_GE` and reuse it? Intuitively, the previous solution to the model should be closer to the next iteration than the solution that I fed into the model in the very beginning.

Thanks for any pointers!

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**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [May 25, 2022, 2:35pm UTC](https://discourse.julialang.org/t/change-variable-outside-of-local-scope/81645/2 "2022-05-25T14:35:25Z")

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I think we cannot answer this without knowing more. `residuals_GE` is not called directly inside `solve_GE` but (I assume) it is called inside `nlsolve`. If `par` is `mutable struct` and `residuals_GE` changes `par`, then these changes will reflect wherever you have the `par` object stored (i.e., you are already updating the parameters). However, if `par` is immutable (plain `struct`), or `residuals_GE` do not mutate it, then things complicate. In this case, if `residuals_GE` do not mutate the old `par` neither return an updated `par`, then there is nothing to do. The new `par` value do not escape `residuals_GE`. If it returns an updated copy, then `nlsolve` would need to be able to deal with this extra return and keep the last `par` returned by `f!` to return it too.

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**Author:** ![IljaK91](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iljak91/32/44301_2.png) [@IljaK91](https://discourse.julialang.org/u/IljaK91)\
**Post date:** [May 25, 2022, 2:55pm UTC](https://discourse.julialang.org/t/change-variable-outside-of-local-scope/81645/3 "2022-05-25T14:55:19Z")

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Thank you for your reply and your understanding is accurate. I was using par as an immutable struct combined with the package `Setfield.jl` to update variables that are related to the solution. However, using `@set! par.x = new_x` withing `residuals_GE` did not actually affect `par.x` for the next iteration run through `nlsolve!`. Then I tried declaring `par` as a `mutable struct` and instead used `par.x = new_x`, which indeed updated the field for the next iteration run through `nlsolve!`. Thanks!

Only question: Is that difference in behavior expected between `immutable struct + Setfield` or `mutable struct + direct assignment` or is that a bug?

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<div class="post-metadata">

**Author:** ![pfitzseb](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pfitzseb/32/45566_2.png) [@pfitzseb](https://discourse.julialang.org/u/pfitzseb)\
**Post date:** [May 25, 2022, 3:09pm UTC](https://discourse.julialang.org/t/change-variable-outside-of-local-scope/81645/4 "2022-05-25T15:09:28Z")

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> [@IljaK91](#):
>
> Only question: Is that difference in behavior expected between `immutable struct + Setfield` or `mutable struct + direct assignment` or is that a bug?

That’s expected. Setfield will create a new struct with a changed field, it cannot modify the original struct – you declared that to be immutable after all.

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<div class="post-metadata">

**Author:** ![IljaK91](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/iljak91/32/44301_2.png) [@IljaK91](https://discourse.julialang.org/u/IljaK91)\
**Post date:** [May 25, 2022, 3:15pm UTC](https://discourse.julialang.org/t/change-variable-outside-of-local-scope/81645/5 "2022-05-25T15:15:07Z")

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Thanks for the great explanation! Did not think much about how Setfield works, just thought immutable plus Setfield makes it less likely that I mess with the struct by accident.